Use of nitrogen from an air separation unit as gas turbine air compressor feed refrigerant to improve power output

Abstract: The work produced by the gas turbine expander (40), is improved by cooling a nitrogen product (86), produced by a cryogenic air separation unit (80), to a subambient temperature and combining this subambient cooled, nitrogen product (92) with the feed air stream (10) prior to compression (20). The cooling can be produced by expanding (90) the nitrogen product (86) and/or saturating it with water (Fig 2;190). The present invention is particularly suited to a process in which at least a portion of the oxygen product (84) produced by the cryogenic air separation unit (80) is reacted with a carbonaceous feedstock (72) in a gasification unit (70) to produce the fuel gas.

Description: The present invention relates to a gas turbine process for the production of work to generate electricity or to drive a device. More specifically, the present invention relates to a process for using the nitrogen from a cryogenic air separation process as a feed gas to the air compressor of the gas turbine in the gas turbine process. Oxygen-blown gasification of a carbonaceous feedstock, for example coal, petroleum coke, residual oil, oil emulsions, and tar sands in gasification combined cycle (GCC) power plants generate a large flow of excess nitrogen associated with the production of oxygen feed for the gasification reaction. One difficulty confronting industry is to make the most efficient use of this effectively free excess nitrogen in the GCC facility. Other related difficulties in GCC technology include the continuing drive for more generating capacity at increasingly greater efficiencies and the imposition of severe limits on NO x emissions to the atmosphere. Further, as existing GCC facilities' capacity, efficiency, and NO x emission rates become inadequate to meet increasingly strict future demands, there is a need for inexpensive retrofitting modifications to meet these new, more stringent requirements. The present invention allows the use of the available excess nitrogen in a GCC power plant to increase power generation capacity and improve power generation efficiency with minimum additional capital costs in both new grass roots plants and in re-powering/ retrofitting applications. There are several methods for utilizing nitrogen in a GCC power plant. The simplest method is to vent the excess nitrogen to the atmosphere. The simple venting of the nitrogen is employed when minimum integration or no integration between the air separation unit and the gas turbine is desired. Another method is described in GB-A-2,067,668. This method consists of returning the excess nitrogen at unspecified temperature to the inlet of the gas turbine air compressor with the sole expressed purpose of reducing NO x generation. This same method is also disclosed in US-A-4,297,842. Still another method is to pass the nitrogen through an auxiliary compressor to inject it into the gas turbine fuel stream which reduces NO x generation by reducing the flame temperature in the combustor. This practice is disclosed in US-A-5,081,845, EP-A-0,137,152 and US-A-4,224,045. This method requires a costly auxiliary compression system, so, implementation is expensive. Also, since nitrogen injection into the fuel stream typically requires nitrogen pressures of 50-150 psi (350-1050 kPa) higher than the 200-250 psia (1400-1700 kPa) combustion pressure, this practice requires excessive compression with much of the energy wasted as pressure drop to the combustor. Direct injection of the nitrogen into the combustor of the gas turbine to reduce NO x is another commonly used method in GCC. US-A-4,729,217, US-A-4,707,994, US-A-4,697,415, US-A-4,651,519, US-A-4,631,915 and CA-A-1,238,195 describe the method and several minor variations thereof. This method does reduce NO x formation, but also requires expensive auxiliary compression to excessive pressures. Some methods involve heating and compressing the nitrogen before injection directly into the gas turbine expander to generate additional motive flow. US-A-5,081,845, US-A-5,076,837, US-A-4,019,314 and US-A-3,731,495 by provide a detailed description of these methods. These methods do presumably improve power generating efficiency, but require expensive additional compression equipment and reduce the thermodynamic efficiency of the main expander. Another scheme for use of the nitrogen stream is described in US-A-4,697,413. This scheme involves using an auxiliary compressor to feed the nitrogen to the coal gasifier to quench the high temperature reaction. Other uses for the nitrogen stream involve synthesis of compounds like ammonia that use the nitrogen as a feed stream. These processes all require expensive additional process equipment and add extreme complexity to the overall process so they will not be described here. Some gasification systems employ a small part of the nitrogen to facilitate dry materials transport, but this only uses a small fraction of the available nitrogen, so these systems and their processes will not be described here. As further background, several processes are known in the art for refrigerating the gas turbine air compressor feed. These processes, which involve expensive, complicated auxiliary equipment, are taught by US-A-3,788,066; US-A-3,796,045; US-A-3,877,218 and US-A-4,424,667. The present invention is an improvement to a process for the production of work to generate electricity or to drive a device using a gas turbine. In the process, feed air stream is compressed and combusted with a fuel gas to produce a combustion product. This combustion product is expanded in a gas turbine expander, thereby producing a hot exhaust gas and work. This produced work is used to generate electricity or to drive a device. The improvement to the process, which increases the work produced by the gas turbine expander, is characterized by cooling nitrogen product produced by a cryogenic air separation unit to a subambient temperature and combining this subambient cooled, nitrogen product with the feed air stream prior to compression. The improvement of the present invention is particularly suited to the process, wherein at least a portion of the oxygen product produced by the cryogenic air separation unit is reacted with a carbonaceous feedstock in a gasification unit to produce the fuel gas, which is rich in carbon monoxide and hydrogen. The carbonaceous feedstock reacted in the gasifier unit can be coal, petroleum coke, tar sand bitumen, tar sand emulsion, municipal wastes, petroleum residua, waste oil or mixtures thereof. The process of the present invention can further comprise a combined cycle option, wherein the hot exhaust gas is cooled to generate steam, which in turn is expanded to generate electricity. There are two primary embodiments of the improvement. The first is applicable to an air separation unit producing a nitrogen product at an elevated pressure, i.e., when the nitrogen product is produced at a pressure at least 3 psi (20 kPa) higher than the pressure of the gas turbine air compressor inlet. In this embodiment, the nitrogen product is expanded, thereby producing a cooled, nitrogen product which is mixed with the feed air entering the gas turbine air compressor inlet. The second embodiment is particularly applicable to an air separation unit producing a nitrogen product at typically low pressure, i.e., when the product nitrogen is produced at a pressure between 0.5 and 3 psi (3 and 20 kPa) higher than the pressure of the gas turbine air compressor inlet, but is also applicable to an air separation unit producing a nitrogen product at an elevated pressure. In the second embodiment, the nitrogen product is cooled and saturated by direct contact with water and the cooled nitrogen product mixed with the feed air entering the gas turbine air compressor inlet. The two improvement embodiments can be both incorporated in the process of the present invention. In other words, elevated pressure nitrogen product is expanded and then further cooled and saturated by contact with water in a cooler/saturator. Figures 1 and 2 are schematic diagrams of two embodiments of the process of the present invention. In order to fully understand the improvement of the present invention, it will be discussed in detail with reference to two embodiments. The improvement to the process, in a first primary embodiment, takes the waste nitrogen stream produced by an elevated pressure air separation unit and passes it through an expander to generate power and to chill the nitrogen stream to subambient temperatures. The chilled stream is then fed to the inlet of the air compressor for the gas turbine, displacing some of the ambient temperature air feed. This modification cools and densifies the gas turbine compressor feed, increasing the total gas throughput of the gas turbine compressor. As a side effect, since the nitrogen stream is depleted in oxygen relative to the normal feed air to the gas turbine compressor, the oxygen content of the oxidant will be reduced, resulting in lower peak flame temperatures that will in turn result in lower NO x generation. A flow diagram of this process is shown in Figure 1. With reference to Figure 1, feed air, in line 10, and chilled, low pressure nitrogen, in line 92, are compressed in gas turbine air compressor 20 to produce a compressed, combustion air stream. This compressed, combustion air stream, in line 22, is then fed to and combusted with fuel gas, in line 62, and, optionally, steam, in line 52, in combustor 30 producing a combustion product stream. This combustion product stream, in line 32, is fed to and expanded in gas turbine expander 40 thereby producing a hot exhaust gas stream and electricity. Typically, gas turbine expander 40 and gas turbine air compressor 20 are mechanically linked. Both the hot exhaust gas stream, in line 42, and steam from coal gasifier 70, in line 74, are fed to heat recovery steam generator 50. It is important to note that although the embodiments of the present invention are being described with reference to use of coal gasifier 70, this gasifier could be one which processes any carbonaceous or other suitable feedstock. The purpose of the heat recovery steam generator is to recover heat from the hot exhaust gas stream and the steam from the coal gasifier, to produce electricity and appropriate level steam for use elsewhere in the process. For example, some of the steam can be used to regenerate the thermal swing adsorbers in air separation unit 80. A warm exhaust gas is vented, via line 58, from heat recovery steam generator 50. In gasifier 70, a carbonaceous feedstock such as coal, in line 72, and oxygen, in line 84, are reacted to produce a synthesis gas rich in hydrogen and carbon monoxide. This synthesis gas comprises the fuel gas, in line 62, used in the combustor. In air separation unit 80, compressed air, in line 82, is cryogenically separated into an elevated pressure, oxygen product and an elevated pressure, nitrogen product. It is important to note in the present invention that the source of the compressed air, in line 82, can be from a stand alone air compressor or in whole or in part from compressor 20. The elevated pressure, oxygen product, in line 84, is fed to coal gasifier 70. The elevated pressure, nitrogen product, in line 86, is expanded in expander 90, thereby producing a chilled, low pressure nitrogen stream, in line 92, while producing work to generate electricity. The preferred range of conditions for the elevated pressure nitrogen stream are: temperature less than 100°F (55°C) above ambient, pressure greater than 3 psi (20 kPa) above the regular gas turbine compressor inlet, and total mass flow less than 20% of the total gas turbine compressor air feed mass flow rate. A typical total gas turbine compressor air feed mass flow rate for a 290 MW GCC system is about 3.3x10 6 lb/h (1.5x10 6 kg/h). The preferred range of conditions for the chilled low pressure nitrogen stream are: temperature less than ambient minus 10°F (5°C) and pressure slightly (<0.5 psi(3 kPa)) greater than the gas turbine compressor inlet. There is no specific stream composition limitation but it preferably consists of nitrogen with lesser amounts of other gases including, but not limited to, H 2 O, CO 2 , Ar, and O 2 . The improvement to the process, in the second primary embodiment, is particularly suited to the process when it utilizes an air separation unit operating at conventional low pressures and where the available nitrogen is not saturated with water. This embodiment uses a saturation cooler to cool the nitrogen stream before it enters the gas turbine compressor inlet. This cooling and saturating has similar effects to those in the first primary application described above. In addition, this saturator/cooler will increase the mass flow of the nitrogen stream and increase its heat capacity, further improving the process. This embodiment may be employed with low or elevated pressure nitrogen streams and, also, in conjunction with the expander embodiment described above. Depending on the relative humidity of the main air feed, the saturator can be run to varying levels of saturation to prevent condensation problems in the gas turbine compressor. The flow diagram for this option (for a case without a nitrogen expander) is shown in Figure 2. Similar process streams and equipment in Figures 1 and 2 share the same identification number and only the differences between the two figures will be described. With reference to Figure 2, the air separation unit 80 provides a low pressure, nitrogen product, in line 86, which is chilled and saturated with water in saturator/cooler 190, thereby producing a chilled, water saturated nitrogen stream, in line 192. The preferred range of conditions for the unsaturated nitrogen stream are: temperature less than 20°F (10°C) above ambient, pressure greater than 0.5 psi (3 kPa) above regular compressor feed, less than 50% relative humidity, and total mass flow less than 20% of gas turbine compressor air feed mass flow rate. As mentioned above, a typical total gas turbine compressor air feed mass flow rate for a 290 MW GCC system is about 3.3x10 6 lb/h (1.5x10 6 kg/h). The preferred range of conditions for the chilled saturated nitrogen stream are: temperature less than ambient minus 10°F (5°C) and pressure greater than regular compressor feed pressure. The water feed to the saturator is preferably at the boiler feed water make up temperature or the cooling water inlet temperature, whichever is colder, and has a gross flow rate of less than 10% of the nitrogen flow rate (<6.6x10 4 lb/h (3x10 4 kg/h) water flow for a 290 MW GCC plant). Any excess water flow can be recycled back to the saturator/cooler inlet since this water is at reduced temperature and thus will more effectively cool the nitrogen stream. Without water recycle, the water flow can provide excess chilled water coolant in the air separation unit. This additional option is shown by flow stream line 194 in Figure 2. The excess water flow can be controlled so as to divide the cooling capacity between the air separation unit and the gas turbine system in the most efficient way. There is no other specific gas stream composition limitation, but the nitrogen stream preferably contains a plurality of nitrogen with lesser amounts of other gases including, but not limited to, H 2 O, CO 2 , Ar and O 2 . The improvement of the present invention also comprises combining the above two specific embodiments. In this case, elevated pressure nitrogen is expanded in an expander to cool the nitrogen and produce electricity and then the cooled, expanded nitrogen is further cooled and saturated by contact with water in a cooler/saturator. The present invention is of value in that it provides increased power generation output at a minimum cost without introducing undesired complexity and unreliability. Table 1 summarizes the advantages of the invention relative to a base case without integration of the nitrogen waste stream as calculated through a computer simulation. The system chosen for simulation consists of a gas turbine, a heat recovery steam generation system, a triple pressure 1450 psig/1000°F/1000°F (10 MPa/540°C/540°C) reheat extraction-induction condensing steam turbine, an integrated coal gasification system, and either an elevated pressure or a low pressure air separation unit (ASU in Table 1). The base case plant is designed for ISO ambient conditions 59°F (15°C), 14.7 psia (101 kPa), and 60% relative humidity, 65°F (18°C) cooling water) and no integration between the air separation unit and the rest of the plant. Summer ambient conditions best demonstrate the increased power capacity of the invention and are defined to be 90°F (32°C) ambient, 14.7 psia (101 kPa), and 50% relative humidity. The primary embodiment with an expander and an elevated pressure air separation unit design, shown in Figure 1, was evaluated under two conditions listed above as Option 1A and 1B. Option 1A shows the benefits of the invention at summer conditions when the total plant efficiency is maximized. For this option, the gas turbine and steam turbine heat rate based on syngas fuel value can be improved from 6001 BTU/kWh (6331 kJ/kWh) to 5922 BTU/kWh (6248 hJ/kWh), or by 79 BTU/kWh (83 kJ/kWh) at a cost of 2.3 MW of capacity reduction. Option 1B shows the benefits of the invention at summer conditions when the total plant power output is maximized. For this option, the total net power output for the plant can be increased by 6.1 MW (2.3%) with essentially no efficiency loss and an incremental capital cost of only 246 $/kW. The performance of the embodiment of the invention with a saturator/cooler and a low pressure air separation design, as shown in Figure 2, is listed above as Option 2. This option was evaluated at summer conditions with maximum cooling sent to the chilled, saturated nitrogen stream. Option 2 has the benefit of increasing total plant net power output by 1.6 MW (0.6%) at summer conditions with an incremental capital cost of only 263 $/kW. The primary advantage of the invention is the increase in summer peak generation capacity at costs significantly less than typical base load costs of about $1500/kW. This is the result of the significant leverage between the relatively low cost to increase gas turbine compressor mass flow and its relatively large effect on total power output. Another advantage of the present invention is that the gas turbine NO x emission level can be reduced from 25 to 9 vppm. Another significant advantage of this invention is that it introduces a minimum additional level of complexity to a non-integrated process since the only integration point is at the gas turbine air compressor inlet. Thus, the entire plant can still be operated at the non-integrated output level even if the equipment used to implement this invention is taken out of service. In the power generating industry, process reliability and minimal unplanned down time have extremely high value, so this feature is a clear advantage. Also, since the implementation of this invention does not significantly disturb the process flow sheet, it can be easily and affordably added as a retrofit to an existing nonintegrated GCC plant.

Claims: A process for the production of work to generate electricity or to drive a device using a gas turbine, wherein a feed air stream is compressed and combusted with a fuel gas to produce a combustion product which is expanded in a gas turbine expander, thereby producing a hot exhaust gas and producing work used to generate electricity or to drive a device, characterized in that a nitrogen product produced by a cryogenic air separation unit is cooled to a subambient temperature, and the subambient cooled, nitrogen product combined with the feed air stream prior to compression thereby increasing the work produced in the gas turbine expander. A process as claimed in Claim 1, wherein the cryogenic air separation unit also produces an oxygen product and at least a portion of said oxygen product is reacted with a carbonaceous feedstock in a gasification unit to produce the fuel gas rich in carbon monoxide and hydrogen. A process as claimed in Claim 1 or Claim 2, wherein the hot exhaust gas is cooled to generate steam. A process as claimed in any one of the preceding claims, wherein the nitrogen product is produced at a pressure at least 20 kPa (3 psi) higher than the regular pressure of the gas turbine air compressor inlet and is expanded to produce the subambient cooled, nitrogen product. A process as claimed in any one of Claims 1 to 3, wherein the nitrogen product is produced at a pressure between 3 and 20 kPa (0.5 and 3 psi) higher than the regular pressure of the gas turbine air compressor inlet and is contacted with water to produce the subambient cooled, nitrogen product. A process as claimed in any one of Claims 1 to 3, wherein the nitrogen product is produced at a pressure between 3 and 20 kPa (0.5 and 3 psi) higher than the regular pressure of the gas turbine air compressor inlet and is expanded thereby producing a cooled expanded, nitrogen product and the cooled expanded, nitrogen product is then contacted with water to produce the subambient cooled, nitrogen product. A process as claimed in any one of Claims 4 to 6, wherein the elevated pressure nitrogen product has a temperature less than 55°C (100°F) above ambient and a total mass flow less than 20% of the total gas turbine compressor air feed mass flow rate and the subambient nitrogen product has a pressure less than 3 kPa (0.5 psi) greater than the gas turbine air compressor inlet and a temperature less than ambient minus 5°C (10°F). A process as claimed in any one of Claims 1 to 3, wherein the nitrogen product is produced at a pressure between 3 and 20 kPa (0.5 and 3 psi) higher than the regular pressure of the gas turbine air compressor inlet and is contacted with water to produce the subambient cooled, nitrogen product. A process as claimed in Claim 6 or Claim 8, wherein the unsaturated nitrogen product prior to contact with water has a temperature less than 10°C (20°F) above ambient, less than 50% relative humidity, and a total mass flow less than 20% of the gas turbine compressor air feed mass flow rate and the subambient nitrogen product has a temperature less than ambient minus 5°C (10°F). An apparatus for the production of work to generate electricity or to drive a device using a gas turbine, said apparatus comprising: a compressor (20) for feed air (10); a combustor (30) for combustion of a fuel gas (62) with said compressed feed air (22) to produce a combustion product (32); an expander (40) for expansion of said combustion product (32) to produce a hot exhaust gas (42) and work for generating electricity or driving a device; and an air separation unit (80), characterized in that the apparatus further comprises: cooling means (90; 190) for cooling to a subambient temperature a nitrogen product (86) produced by said cryogenic air separation unit (80); and means (92) for combining said subambient cooled, nitrogen product with said feed air (10) prior to supply to the compressor (20). An apparatus as claimed in Claim 10, further comprising: a gasification unit (70); and means (84) for supplying to said gasification unit (70) at least a portion of an oxygen product from said cryogenic air separation unit (80) for reaction with a carbonaceous feedstock (72) to produce said fuel gas (62). An apparatus as claimed in Claim 10 or Claim 11, further comprising a heat recovery steam generator (50) in which said hot exhaust gas (42) is cooled to generate steam. An apparatus as claimed in any one of Claims 10 to 12, wherein the air separation unit (80) supplies said nitrogen product at a pressure greater than 20 kPa (3 psi) higher than the regular inlet pressure of the compressor (20) and said cooling means comprises an expander (90). An apparatus as claimed in any one of Claims 10 to 13, wherein the air separation unit (80) supplies said nitrogen product at a pressure greater than 20 kPa (3 psi) higher than the regular inlet pressure of the compressor (20) and said cooling means comprises a cooler/saturator (190) in which said nitrogen product is contacted with water. An apparatus as claimed in any one of Claims 10 to 12, wherein the air separation unit (80) supplies said nitrogen product at a pressure between 3 and 20 kPa (0.5 and 3 psi) higher than the regular inlet pressure of the compressor (20) and said cooling means comprises a cooler/saturator (190) in which said nitrogen product is contacted with water.

Inventors: FLEMMING FREDERICK CHARLES, SCHARPF ERIC WILLIAM, FLEMMING, FREDERICK CHARLES, SCHARPF, ERIC WILLIAM
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1994
Main classification: F02C7
IPC codes: F25J3, F02C7, F01K23, F02C3